1. Field of the Invention
The present invention relates to grounding techniques and leakage current detection techniques for lamps associated with LCD display panels. Also, this invention relates to the reliable ignition technique for each lamp in a multiple lamp system. In particular, the present invention relates to a common grounding technique utilizing the metallic chassis of the LCD panel to insure that leakage current of individual lamps is detected to provide more accurate current feedback control. Particular utility for the present invention is in LCD display panels with multiple CCFL lamps, although the present invention is equally applicable in single lamp and multiple lamp environments including lamps other than CCFL type.
2. Description of Related Art
FIG. 1 depicts a conventional Royer circuit that is used to drive multiple CCFL lamps shown as CCFL1 . . . CCFLN. The operational characteristics of the controller and the Royer Is circuit comprised of switches Q1 and Q2 on the primary side of the transformer T1 are well known to those skilled in the art. Each of the CCFLs, CCFL1 . . . CCFLN, and in particular the bottom portions thereof are tied together and are coupled to the rectifier diodes and across the sense resistor R to a virtual ground as depicted. Importantly, the secondary side of the transformer T1 is also coupled to the circuit virtual ground. Virtual grounding techniques are widely used in panel manufacturing and dc/ac inverter manufacturing, since virtual grounding takes advantage of lower potentials needed for a ground without the need for additional wiring. However, the drawback to using virtual grounding techniques is that the leakage current developed within the lamp cannot be detected through the current feedback loop across Rs. As is well understood in the art, CCFLs may be equivalently drawn as lumped parasitic capacitance to represent the parasitic capacitor between the CCFLs and the LCD display panel.
FIGS. 2A and 2B depict the secondary side operation of the circuit of FIG. 1 during the negative and positive half-cycles, respectively, of the AC signal developed by the transformer. In FIG. 2A, since the bottom of the secondary side of the transformer is tied to chassis ground, the total current IT flows through the bottom of the lamp. Importantly, the leakage current ILk (along the current path represented by leakage capacitor CLk, also tied to ground) flows out of the lamp. At the top of the lamp, the current going back to the top of the transformer is reduced by the value of the leakage current, i.e., ITxe2x88x92ILk. Thus, the controller cannot regulate the lamp based on the total current that includes the leakage current, since this current is lost to ground. Similarly, in FIG. 2A (positive half cycle), the feedback current developed across the sense resistor is reduced by the leakage current.
Thus, there exists a need for a circuit construction that ensures that lamp regulation is based on the total current delivered to the load, inclusive of the leakage current.
Accordingly, one embodiment of the present invention provides a common ground circuit, comprising: a transformer having a primary side and a secondary side, said secondary side having a top and a bottom; a lamp having a top, a bottom and a leakage current path, said lamp coupled in parallel to said secondary side of said transformer, said top of said secondary side electrically coupled to said top of said lamp; a common ground connected to a ground node; and a current feedback loop generating a current feedback signal proportional to the total current delivered to said lamp; said bottom side of said transformer electrically coupled to said ground node through said current feedback loop, said bottom of said lamp electrically coupled to said ground node and said leakage current path electrically coupled to said ground node.
The present invention also provides a lamp regulating circuit, comprising a DC to AC inverter circuit generating and an AC signal from a DC source; and a secondary-side transformer circuit, comprising: a transformer having a primary side and a secondary side, said secondary side having a top and a bottom; a lamp having a top, a bottom and a leakage current path, said lamp coupled in parallel to said secondary side of said transformer, said top of said secondary side electrically coupled to said top of said lamp; a common ground; and a current feedback loop generating a current feedback signal proportional to the total current delivered to said lamp; said bottom side of said transformer electrically coupled to said common ground through said current feedback loop, said bottom of said lamp electrically coupled to said common ground and said leakage current path electrically coupled to said common ground.
Still further, the present invention provides a transformer circuit, comprising: a transformer having a primary side and a secondary side, said secondary side having a top and a bottom; a lamp having a top, a bottom and a leakage current path, said lamp coupled in parallel to said secondary side of said transformer, said top of said secondary side electrically coupled to said top of said lamp; a common ground connected to a ground node; and a current feedback loop generating a current feedback signal proportional to the total current delivered to said lamp; said bottom side of said transformer electrically coupled to said ground node through said current feedback loop, said bottom of said lamp electrically coupled to said ground node and said leakage current path electrically coupled to said ground node; wherein at least a portion of energy delivered by said transformer causing said total lamp current to flow from said bottom of said transformer through said current feedback loop to said common ground, a lamp current portion of said total lamp current flows from said common ground through said lamp, and a leakage current portion of said total lamp current flows from said common ground through said leakage current path.
In method form, the present invention provides a common grounding method, comprising: coupling a top side of a transformer to a top side of a lamp; coupling a common ground to a ground node; and coupling a feedback loop circuit to a bottom side of said transformer coupled and to said ground node; coupling the bottom of said lamp said ground node; and coupling a leakage current path from said lamp to said ground node.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments and methods of use the present invention is not intended to be limited to these preferred embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited as only set forth in the accompanying claims.
Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein: